May 8, 2014

AGU Journal Highlights — May 6, 2014

by editor

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research: Earth Surface (JGR-F), Journal of Geophysical Research: Oceans (JGR-C), and Water Resources Research (WRR).

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://onlinelibrary.wiley.com/ and inserting into the search engine the full doi (digital object identifier), e.g. doi: 10.1002/2013WR014696. The doi is found at the end of each Highlight below.

Journalists with AGU press subscriptions may access and download the papers cited in this release by clicking on the links below. If you are a reporter and have not yet registered for a complimentary press subscription, please fill out the form at http://news.agu.org/agu-press-subscriptions/.

A hexagon-shaped atmospheric phenomenon first spotted on Saturn by Voyager 1 and 2 has intrigued scientists since the 1980s. More recently, NASA's Cassini mission has periodically observed the hexagon, a strong eastward jet that rotates at 120 meters per second (394 feet per second). Scientists believe that the persistence of the hexagon could put to rest the oft-debated question of the length of Saturn's rotational period.

Sánchez-Lavega et al. present an estimate (in Earth hours) of the length of Saturn's day based on data from Cassini and ground-based images. They tracked the movement of Saturn's hexagon for 5.5 years and find that despite large radiative forcing in Saturn's atmosphere, the rotation of the hexagon remained constant. These findings suggest that the hexagon is deeply rooted within Saturn's atmosphere and that its rotational period could reveal the rotational period of Saturn's internal solid body, which the authors estimate as 10 hours, 39 minutes, and 23.01 seconds, plus or minus 0.01 seconds.

A. Wesley: International Outer Planet Watch, Planetary Virtual Observatory Laboratory, Bilbao, Spain; and Astronomical Society of Australia, School of Physics, University of Sydney, Sydney, New South Wales, Australia.

2. Antarctica's Whillans Ice Plain ice flows are highly variable

The Whillans Ice Plain (WIP) is a roughly 20,000-square-kilometer (772-square-mile) region of the West Antarctic Ice Sheet that acts as a massive conveyor, driving glacier ice into the Ross Ice Shelf. As the climate changes, knowing how large bodies of ice like the WIP behave will be important to assessing sea level rise.

Since measurements first began in 1963, researchers have found that the ice flows in the WIP have been slowing down. Based on what were initially decadal observations, researchers calculated that the slowdown was occurring at a constant rate.

Starting in 2007, Beem et al. began collecting annual measurements on the WIP. The authors find that rather than undergoing a steady deceleration, ice flow rates fluctuate on interannual timescales. The ice flow rate is controlled by a range of resistive stresses, such as side drag, basal drag, and other factors, but previous research, combined with the current findings, suggests that changes in basal drag are responsible for the WIP's interannual variability.

What would cause this increase in basal drag? Water loss from the surface beneath the ice—caused by water freezing to the glacier's underside, or by subglacial streams being diverted along a new path, the authors speculate. Both mechanisms are supported by existing research. The authors note that changes to ice flow at WIP are linked to changes in internal glacier dynamics rather than to climate variability.

M.A. King: School of Geography and Environmental Studies, University of Tasmania, Hobart, Tasmania, Australia; and School of Civil Engineering and Geosciences, University of Newcastle, Newcastle upon Tyne, UK;

3. Climate change, water rights, and agriculture: A case study in Idaho

Water supply is important to agriculture and other consumptive uses in the arid and semiarid climate zones, but it has become increasingly uncertain under a changing climate. More importantly, how agricultural output will be affected may depend on how water resources are allocated based on the dominant sharing rule, according to Xu et al., who conducted a case study in Idaho. In the western United States, climate models predict a large warming trend in climate, which will lead to a reduced amount of snowmelt-driven summer and fall streamflows and increased volatility in irrigation water supply during warm growing seasons. In Idaho, water resources are appropriated according to the Prior Appropriation Doctrine, a first-come, first-served water sharing rule by which local water institutions allocate more water resources to farms with water entitlements that have been in existence longer.

The researchers analyzed how irrigated farmers' land use changed in response to water supply information by using data on farmland use and projections of water supply under a warming climate, including projected seasonal variation. They find that if climate change increases the volatility of the temperature and the water supply, irrigated agriculture in the region could face significant damages. In fact, crop revenue losses could be up to 32 percent, the authors predict. Their analysis also indicates that farmers' priority in water rights affects how they respond to changes in water supply. The authors suggest that both the use of groundwater and a free-market allocation system can help mitigate climate change-induced economic losses. But both these two mitigation methods have environmental and institutional constraints that will potentially limit their ability to reduce the negative effects of climate change.

In San Francisco, as with other places, climate change is threatening to upset the balance the city has struck with the local hydrology. Precipitation in Northern California has already been increasing in winter and decreasing in summer, and projected changes to the behavior of the El Niño–Southern Oscillation hint at more of the same.

While any extra rain may sound like good news for a state currently plagued by drought, the projected increase in extreme winter precipitation events would also cause a host of problems—particularly in urban centers. Pavement and buildings increase overland flow, raising the potential for flash flooding and driving surface pollutants into the water supply. To mitigate against these issues urban planners have turned to "low impact development," a suite of best practices and tools, such as building infiltration trenches and strategically placing vegetation, to capture, filter, and slow the water down.

A suspected side benefit of low impact development is that it could also work to boost groundwater recharge rates. Using experimental plots at San Francisco State University, Newcomer et al. tested how a low impact development-style infiltration trench compared against an irrigated lawn when it came to turning water inputs into groundwater. The authors find that the recharge efficiency of the infiltration trench, at 58 percent to 79 percent, was much higher than that of the lawn, at 8 percent to 33 percent.

Though groundwater will certainly be beneficial during San Francisco's increasingly dry summers, sustaining groundwater stores in coastal cities is particularly important long term. As the sea level rises, salt water threatens to infiltrate coastal aquifers. Only by maintaining sufficient groundwater stores can this important freshwater source be maintained.

The accumulation and melting of sea ice in the Arctic has an enormous impact on the local climate, which in turn can affect the global climate. As the climate warms and Arctic sea ice retreats, it has become crucial to understand the complex ice-atmosphere-ocean dynamics within the Arctic. One major component in this dynamic is the Beaufort Gyre (BG), a wind-driven sea ice circulation and freshwater reservoir in the Arctic's Beaufort Sea.

The BG is a notoriously dangerous area to observe because of its hostile conditions. Working around this, Krishfield et al. set out to investigate recent rapid sea ice decline in the BG, using data already collected between 2003 and 2012. The authors used data from moorings, ship-based surveys, and satellite radiometers to estimate ice thickness, which, when combined with satellite estimates of ice extent, can give an estimate of sea ice volume.

The authors find a net sea ice decline over the 9 years studied, with record minima of ice volume in 2007 and 2012. Freshwater export from this region during the last 3 years was also observed. These data indicate that an anticyclonic climate regime that has been persistent in the BG since the late 1990s may be weakening, which may lead to a relatively warmer and wetter climate locally, but could produce cooling in the North Atlantic.

Salt marshes are coastal habitats that store important nutrients and serve as shelter for many estuarial species. These habitats are threatened by rising seas and human expansion, so it has become increasingly important to improve models of how these habitats degrade.

There are many ways that salt marshes retreat naturally, including surface erosion or widespread slumping into the sea, via processes such as sliding or toppling. Studying the way the edge of a salt marsh erodes is important for determining morphological changes in the marsh, but, so far, no mechanical models exist that describe how toppling occurs.

Bendoni et al. present the first model for toppling induced by wind waves—waves of water pushed by the wind. The authors conducted laboratory experiments to investigate the effects of wind waves on salt marsh edges and find that failure occurs when water fills tension cracks, putting stress on the soil that causes it to fail. The authors then created a model based on their findings that replicates lab results well. These models may help improve salt marsh management strategies.